Subscriber access provided by Caltech Library Article The Role of Nitrogen Dioxide in the Production of Sulfate during Chinese Haze-Aerosol Episodes Lijie Li, Michael R Hoffmann, and Agustin J. Colussi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05222 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018
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1 The Role of Nitrogen Dioxide in the Production of Sulfate during Chinese Haze-Aerosol 2 Episodes 3 4 Lijie Li, Michael R. Hoffmann and Agustín J. Colussi* 5 Department of Environmental Science & Engineering, California Institute of Technology, 6 Pasadena, CA 91125, United States 7 8
9 Haze events in China megacities involve the rapid oxidation of SO2 to sulfate aerosol. Given the
10 weak photochemistry taking place in these optically thick hazes, it has been hypothesized that
11 SO2 is mostly oxidized by NO2 emissions in the bulk of pH > 5.5 aerosols. Since NO2(g)
12 dissolution in water is very slow and aerosols are more acidic, we decided to test such
+ 13 hypothesis. Herein, we report that > 95% NO2(g) disproportionates: 2 NO2(g) + H2O(l) = H +
14 NO3 (aq) + HONO (R1), upon hitting the surface of NaHSO3 aqueous microjets exposed to
15 NO2(g) for < 50 s, thereby giving rise to strong NO3 (m/z = 62) signals detected by online
16 electrospray mass spectrometry, rather than oxidizing HSO3 (m/z = 81) to HSO4 (m/z = 97) in
17 the relevant pH 3 6 range. Since NO2(g) will be consumed via R1 on the surface of typical
18 aerosols, the oxidation of S(IV) may in fact be driven by the HONO/NO2 generated therein.
19 S(IV) heterogeneous oxidation rates are expected to primarily depend on the surface density and
20 liquid water content of the aerosol, which are enhanced by fine aerosol and high humidity.
21 Whether aerosol acidity affects the oxidation of S(IV) by HONO/NO2 remains to be elucidated.
22 23 es-2017-05222n-R2
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24 25 26 TOC 27 28
29
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30 1. Introduction
31 Chinese megacities often experience haze events (HE)1 7 that severely impair visibility and
32 induce acute health effects.5 6, 8 11 Hazes mainly consist of sulfate aerosols produced in the
33 atmospheric processing of SO2 and NO2 emissions under particularly adverse meteorological
34 conditions.12 14 Current models of chemistry in HE significantly underestimate sulfate formation
15 17 35 revealing that the mechanism of SO2 oxidation is not well understood. The drastic
18 21 36 attenuation of actinic radiation in hazes, suggests that SO2 is oxidized by NO2(g) in
37 heterogeneous processes on the aerosol itself, rather than in the gas phase by photogenerated
38 oxidants. The details of such process are uncertain.
39 Known SO2 atmospheric oxidation pathways include gas phase reactions with OH radicals
22 40 and stabilized Criegee intermediates, and aqueous phase reactions with O3, H2O2, organic
41 peroxides, NOx, as well as autoxidation catalyzed by transition metal ions. The low
18, 23 30 42 concentrations of photogenerated oxidants implies that the chemistries of SO2 and NO2 in
43 HE are intertwined. These observations have led to hypothesizing that HSO3 (aq) is rapidly
44 oxidized by the NO2(g) dissolved in aqueous aerosol phases assumed to be at pH > 5.5, reaction
45 R0:
+ 46 2 NO2(aq) + HSO3 (aq) + H2O(l) = 2 H (aq) + HSO4 (aq) + 2 NO2 (aq) (R0)
47 on the basis of R0 rates measured in bulk water.31, 32, 33 Good agreement between field
48 observations and model results on sulfate aerosol formation in HE could be obtained by
49 assuming that R0 proceeds on the surface pH > 5.5 aqueous aerosols at the rates previously
50 reported at pH ∼ 6 in bulk water.31 34 However, the assumptions that aerosols are at pH > 5.5, and
51 NO2(g) dissolves in large surface to volume aerosol droplets as NO2(aq) as it does in bulk water
52 may not apply.35 45 Most recent studies suggest that HE aerosols are in fact in the pH 3 5
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46 52 53 range. Previous laboratory experiments have shown that collisions of ppm NO2(g) (i.e., in the
54 absence of N2O4) with the surface of aqueous electrolyte solutions (but not on the surface of pure
55 water) yield NO3 (aq) via a first order in [NO2] hydrolytic disproportionation catalyzed by
56 anions, reaction R1.42, 45
+ 57 2 NO2(g) + H2O(l) = H + NO3 (aq) + HONO (R1)
1 58 The facts that NO2(g) is weakly soluble in pure water (Henry’s law constant H ∼ 0.01 M atm )
59 and its uptake coefficient on pure water very small: γ ∼ 1 × 10 7,53 its dissolution) in water is
60 unfavorable both by kinetic and thermodynamic reasons. The aqueous phase of most
61 atmospheric aerosols, however, is not pure water. In 2009, we found that anions greatly enhance
62 NO2(g) uptake on water. The rate determining step involves the capture of NO2(g) by anions X
42, 45 63 as X NO2 at air aqueous interfaces, followed by the reaction of X NO2 with a second
3 4 64 NO2(g). This phenomenon accounts for the outstanding discrepancy (by a ∼ 10 factor) between
41, 53 65 the NO2(g) uptake coefficients measured in neat water vs those determined on NaCl seeded
66 droplets in a cloud chamber.35, 39, 54 Since HE aerosols naturally contain organic and inorganic
67 anions, the expectation was that the fate of NO2(g) would be determined by R1 at the relevant
68 aerosol air aqueous interfaces.
69 Given the societal and economic impact of HE, we deemed important to elucidate the actual
70 role of NO2(g) in the production of sulfate aerosol under relevant conditions. Herein we report
71 experiments in which aqueous 1 mM NaHSO3 microjets (containing 3 mM of added EDTA to
72 inhibit the potential autoxidation of HSO3 catalyzed by pervasive transition metal ions) ejected
73 from a stainless steel syringe are exposed to 5 ppm NO2(g) for ≤ 50 s in 1 atm of N2(g) at 298
74 K. Reactant and product ions formed on the outermost water layers of the liquid microjets are
75 detected within 1 ms by online electrospray ionization mass spectrometry (o ESI MS). This
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76 technique has been used in our laboratory to investigate a suite of gas liquid reactions at the air
55 61 77 water interface. The analysis of our experimental results focuses on the fate of NO2(g), and
78 the competition between R0 vs R1 during NO2(g) collisions with the surface of aqueous HSO3
79 solutions under conditions relevant to HE. To our knowledge, this is the first study to provide
80 direct experimental evidence on the extent of S(IV) oxidation by NO2(g) on the surface of
81 aqueous electrolyte solutions over a wide pH range.
82 2. Methods
83 Reactive interactions of NO2(g) with HSO3 (aq) are investigated on the fresh surface of
84 continuously flowing HSO3 (aq) microjets that are crossed by NO2(g)/N2(g) beams in the spray
85 chamber of an electrospray ionization (ESI) mass spectrometer maintained at 1 atm of N2(g), and
86 298 K (Agilent 1100 Series G2445A Ion Trap LC MS MSD) (Figure 1). This experimental
87 setup has been described in more detail in previous reports from our laboratory.44 45, 55, 57 58, 60 64
88 Aqueous 1 mM NaSO3H solutions (containing 3 mM EDTA to chelate pervasive traces of
−1 89 transition metal ions known to catalyze HSO3 autoxidation) are pumped at 50 L min through
90 an electrically grounded stainless steel needle injector (100 m bore). These liquid microjets are
91 intersected by beams of NO2(g) diluted in N2(g) at controlled flow rates (MKS). Gas liquid
92 encounters take place during τ ≤ 50 s contact times, which correspond to the estimated lifetimes
93 of the intact microjets prior to their breakup by the nebulizing gas. The outermost layers of the
94 liquid microjets issuing (at 11 cm s 1) from the tip of the syringe are pneumatically stripped and
1 95 nebulized into charged microdroplets by N2(g) flowing at > 250 m s through a coaxial sheath.
96 Anions contained in the charged microdroplets are detected by online ESI mass spectrometry in
97 the m/z 50 100 range. Anion detection was optimized by setting the drying gas temperature at
98 325 °C, and the capillary voltage at 3250 V. Fresh solutions were prepared with Milli Q water
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99 (18.2 M ·cm at 25 °C) that had been sparged with N2(g) for 8 h to remove dissolved O2 (except
100 as indicated) within 5 minutes prior to injection, in a glove box at < 1 ppm O2(g). The pH of
101 solutions was adjusted by adding NaOH(aq) or HCl(aq) and measured with a calibrated pH
102 meter. Throughout, reported pH values correspond to those measured in the bulk of solutions.
103 Sodium bisulfite solution (40% w/w, Sigma Aldrich), hydrochloric acid (> 30% w/w, Sigma
104 Aldrich), sodium hydroxide (≥ 99.0 %, Sigma Aldrich), and ethylenediaminetetraacetic acid
105 (EDTA, > 99.0 %, Sigma Aldrich), and 48.75 ± 2 % ppm NO2(g) in N2(g) (Airgas), were used as
106 received. The actual NO2(g) concentration at the surface of the aqueous microjets is 10 times
107 smaller (5 ppm) due to dilution by the nebulizer gas.
108 109 110 Figure 1. Schematic diagram of the experimental setup. MFC is the mass flow controller. 111 112 3. Results
- 113 3. 1 HSO3 mass spectral intensities on aqueous surfaces
114 HSO3 m/z = 81 mass spectral signal intensities (I81, normalized to their maximum value,
115 I81,max) measured by o ESI MS at the gas aqueous interface of 1 mM NaHSO3 aqueous microjets
116 as functions of bulk pH are shown as blue datapoints in Figure 2. The red trace corresponds to
117 [HSO3 ]/[HSO3 ]max values calculated from reported acidity constants in bulk water: pKa1
+ = + 118 (H2O·SO2 ⇌ HSO3 + H ) = 1.8, pKa2 (HSO3 ⇌ SO3 + H ) = 7.2. Experimental mass spectral
119 data clearly display HSO3 deficits at pH < 5 relative to calculated values, which are due to SO2
120 losses to the gas phase. This is considered a feature particular to the HSO3 /SO2 system because
121 the titration curves of less volatile acids and bases determined in this setup were in accordance
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58, 65 122 with bulk pKa values. The experimental HSO3 signal intensities vs pH data of Fig. 2 provide
123 the reference for analyzing the extent of S(IV) oxidation in the following sections.
124
− 125 Figure 2. Normalized m/z = 81 (HSO3 ) signals measured by o ESI MS on the surface of − 126 aqueous 1 mM NaHSO3(aq) microjets as a function of bulk pH (blue symbols and line). HSO3 + = + 127 mole fractions calculated from pKa1 (H2O·SO2 ⇌ HSO3 + H ) = 1.8, pKa2 (HSO3 ⇌ SO3 + H ) 128 = 7.2 in bulk water (red line).
129 3. 2 NO2(g) reactions on aqueous electrolyte surfaces
130 Mass spectra acquired before and during exposure of pH ∼ 5 (1mM HSO3 + 3 mM EDTA)
131 aqueous microjets to 5 ppm NO2(g) for ≤ 50 s are shown in Figure 3. The main feature is the
132 appearance of a strong NO3 m/z = 62 signal, in contrast with the minimal variations displayed
133 by both HSO3 m/z = 81 and HSO4 m/z = 97 signals upon NO2(g) exposure. This outcome
134 means that NO2(g) molecules hitting the surface of pH ∼ 5 HSO3 microjets mainly undergo fast
135 (within 50 s) disproportionation, leaving barely any NO2 for diffusing into the bulk liquid and,
136 supposedly, participate in R0. The following section explores the effect of pH on the competition
137 between R0 and R1.
138
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139 140 141 Figure 3. ESI mass spectra of the surface of aqueous 1 mM NaHSO3 + 3 mM EDTA, pH ~ 5 142 microjets before and after exposure to 5 ppm NO2(g). Note the presence of minor HSO4 (m/z = 143 97) impurities in the initial solutions. 144 145 146
147 148 Figure 4. Red symbols, line and 95% confidence band correspond to the ratio of HSO3 /HSO4 = 149 I81/I97 signal intensities on the surface of (1 mM NaHSO3 + 3 mM EDTA in N2(g) sparged MQ 150 water) microjets as a function of bulk pH. Blue symbols: after exposure to 5 ppm NO2(g) for < 151 50 s. 152 153
154
155
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156 3. 3 The oxidation of S(IV) on aqueous surfaces
157 The extent of HSO3 (aq) oxidation by NO2(g), expressed as the ratio of [HSO3 ]/[HSO4 ] ∝
158 I81/I97 signal intensities, as a function of pH is shown in Figure 4. I81 values correspond to
159 measured I81 signals corrected for the depressing effect of the NO3 simultaneously produced via
63, 66 160 R1 at the air aqueous interface (see below). Figure 4 clearly shows that the extent of HSO3
161 (aq) oxidation via R0 is minimal throughout, barely exceeding experimental error in the pH 3 6
162 range relevant to HE.
163 We quantified the fraction of NO2(g) that oxidizes HSO3 (aq) at the air aqueous interface as
164 ƒR0, defined by equation E1. In E1, 81' is calculated from the decrease of measured I81 signals,
165 81, corrected for the NO3 depressing effect mentioned above, which is multiplied by the ratio: β
166 = I62/I82, of the o ESI mass spectral measured in (1 mM HSO3 + 1 mM NO3 ) equimolar
167 solutions at each pH to convert I81 decrements into equivalent 62 NO3 changes.
∆ 168 = (E1) ∆ ∆
169 We found that ƒR0 increases from < 6 % within pH 3.0 and 6.0 up to 42% at pH 10.8. It also
170 increases with acidity up to 16 % at pH 2.5 (Figure 5). This finding means that the anion
171 catalyzed hydrolytic disproportionation of NO2(g) on aquated aerosol surfaces, reaction R1, will
172 outcompete R0 throughout, particularly under atmospherically relevant acidic conditions.35, 39, 41,
43, 45, 57 173 We have shown that most electrolytes increase the uptake coefficient of NO2(g) from γ <
174 1.0 × 10 7 in pure water,37 up to γ = 1 × 10 3 1 × 10 4 at air aqueous electrolyte interfaces.35, 39, 41,
45, 57 175 The mechanism of enhancement involves trapping NO2(g) by X as X NO2 at the air water
45 176 interface, which can react further with NO2(g). This interfacial process is expected to dominate
177 the fate of NO2(g) during HE due to the large surface to volume ratio of aerosol microdroplets.
178 We have previously shown that the decay of NO2(g) on aqueous aerosols via reaction R1
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179 accounts for the similar ∼ 4 h NO2(g) decay half lives measured by satellite sightings of urban
180 plumes over world megacities ranging from Singapore to Moscow latitudes.42 We pointed out
181 that if, as generally assumed, NO2(g) were removed by gas phase OH radicals via: NO2(g) +
182 ⋅OH(g) = HNO3(g), much longer decay half lives should have been observed in winter and at
183 high latitudes, given that “OH radicals follow the sun”.67
184 Next, we performed experiments in which we analyzed, within 5 min via o ESI MS, freshly
185 prepared HSO3 solutions in air saturated water without added EDTA.We detected SO3• m/z =
186 80 signals that increase linearly with pH (Figure S3). The formation of SO3• is ascribed to the
187 autoxidation of HSO3 catalyzed by the omnipresent traces of transition metal ions in our
188 solutions.68 Whether this process makes a significant contribution to the oxidation of S(IV) under
189 HE conditions is the subject of further studies.69 70 Additonal experiments carried out by
190 exposing such HSO3 solutions (in air saturated deionized water without added EDTA) to NO2(g),
191 Figure S4, led to I81/I97 ratios similar to those of Figure 4 results NO2(g), meaning that NO2
192 makes negligible contributions to S(IV) autoxidation in our 50 s timeframes.
0.5 0.4
0.3 R0 f 0.2 0.1
0.0 2 4 6 8 10 193 pH 194 195 Figure 5. The fraction of NO2(g) that contributes to S(IV) oxidation, (equation E1) as a 196 function of pH.
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197 4. Discussion
198 4. 1 Indirect role of NO2(g) in the oxidation of S(IV)
199 Our findings open up new perspectives on the mechanism of S(IV) oxidation by NO2(g)
200 during HE. They underscore the fact that the stoichiometry of R0 could not account for the
32 33 201 formation of NO3 in HE. This is so because if NO2(g) were the only available oxidant and,
202 moreover, it were consumed in R0, N(IV)O2 could not be oxidized to N(V)O3 . Any valid
203 mechanism should explain: (1) how are both S(IV)O2 and N(IV)O2 oxidized to HS(VI)O4 and
204 N(V)O3 , and (2) how is HONO produced.
205 We analyzed the fate of NO2(g) by considering its reactive uptake on the aerosol via R1 and
206 its photolysis by scattered solar radiation via R2 under representative HE conditions.
3 207 NO2(g) + hν (λ ≤ 420 nm) = O( P) + NO (R2)
208 Rate constants for the reactive uptake of NO2(g), kR1, were estimated with equation E2 from the
209 kinetic theory of gases.
210 kR1 = ¼ γ vNO2 (S/V) (E2)
2 −1 3 4 211 where vNO2 = 3.7 × 10 m s is the mean thermal speed of NO2(g) at 298 K, γ = 10 10 is the
212 estimated range of the reactive uptake coefficient of NO2(g) on the surface of aqueous electrolyte
213 solutions,45 and S/V (in m2 m 3) is the surface density of aerosols. Aerosols consist of
214 submicron particles with S/V values up to ∼2 ×10 3 m 1 during hazy days, and values ∼ 5 times
215 smaller in clear days.33 By assuming that γ = 10 3, S/V = 2 ×10 3 m 1 and 4 ×10 4 m 1, we
4 1 5 1 216 estimate: kR1 ~ 2 ×10 s and kR1 ~ 4 ×10 s on hazy and clear days, respectively.
217 The low O3(g) concentrations measured during HE indicate that the photolysis of NO2(g),
3 3 218 which generates the O( P) atoms involved in O3(g) formation: O2 + O( P) + M = O3 + M, is
219 much reduced due to the severe attenuation of actinic sunlight during HE. We estimated time
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220 and space averaged photolysis rate constants, kR2, by using the National Center for Atmospheric
221 Research Tropospheric Ultraviolet Visible (TUV) Radiation Model with environmental
222 parameters within the ranges reported for aerosol optical properties and aerosol radiative forcing
24 25 223 parameters (see S.3). Our estimates: (kR1)h/(kR2)h ∼ 0.5 1.2, (kR1)c/(kR2)c ∼ 0.01 0.1 (h = hazy,
224 c = clear), are semi quantitatively consistent with the above premise, and illustrate how the
225 competition between R1 and R2 shifts from hazy to clear days. (See S.3). However, we consider
226 that TUV calculations on the competition between R1 and R2 only provide a lower bound to
227 (kR1)h/(kR2)h, because NO2(g) is not expected to be uniformly distributed but accumulate in the
228 lower layers of dense hazes, where there is minimal actinic radiation and photochemical activity.
229 Together, our estimates and experimental results support the view that the leading pathway
+ 230 for NO2(g) during HE is its heterogeneous disproportionation into (NO3 + HONO/NO2 + H )
231 via R1. Note that R1 is autocatalytic because it ultimately contributes to increase the mass and
232 S/V of the aerosols on which it takes place. The rapid development of HE is in fact consistent
233 with such an autocatalytic process. The fact that relative humidity increments are tracked by
2 3 234 increased particle number concentrations and surface area density S/V ( m m ) of PM2.5,
235 particularly in the accumulation mode, also support the notion that the liquid particles present
236 under such conditions grow from autocatalytic heterogeneous processes.71
237 R1 accounts for the direct formation of NO3 and HONO, and would also account for the
238 formation of HSO4 if the HONO/NO2 produced in R1 could ultimately oxidize S(IV) (see
72 78 239 Figure 6). As a result, since ƒR0 ≤ 0.1 in the realistic pH 3 6 range, i.e., nearly independent
240 of pH (Figure 5), the acidity/basicity of HE aerosols could be a relevant parameter to S(IV)
241 oxidation rates if subsequent processes were to depend on aerosol pH. 79 80 Therefore, our
242 conclusion that the main pathway for NO2(g) in HE is R1 will stand regardless of future
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243 assessments of aerosol pH. The parameters that may affect a heterogeneous process such as R1
244 could be a combination of aerosol S/V and aerosol liquid water content, both of which depend on
245 relative humidity.79 80 We suggest that the apparent dependence of sulfate formation rates on
246 ammonia neutralization of aerosol particles may be the result of cross correlations between
247 aerosol acidity and the actual causal parameters. In this connection, we wish to point out that the
248 enhancing “ammonium” effect on the oxidation rates of SO2(g) by NO2(g) on aqueous solutions
249 might be due to the presence of HSO4 anions that capture NO2(g) at the aerial interface, as
250 explained above.33 It is also important to realize that Clifton et al., results on the homogeneous
251 reaction rates of HSO3 (aq) with the NO2(aq) produced in situ within bulk solution via NO2 (aq)
252 radiolysis,54 are not applicable to the heterogeneous processes being considered here, which
253 involve NO2(g) as reactant.
- 254 4. 2 HONO/NO2 as S(IV) oxidizers
255 Aqueous HONO and NO2 can oxidize HSO3 to HSO4 in the pH 3 5 range of HE aerosols.
+ + 256 HONO and NO2 reductions to NO: E°(HONO + H + e = NO + H2O) = 0.75 V, E°(NO2 + 2 H
+ 257 + e = NO + H2O) = 1.08 V or to N2O: : E°(2 HONO + 4 H + 4e = N2O + 3 H2O) = 1.06 V, E°(2
+ 258 NO2 + 6 H + 4e = N2O + 3 H2O) = 1.04 V, could also drive the thermal (dark) oxidation of
+ 259 HSO3 to HSO4 : E°(HSO3 + H2O = HSO4 + 2 H + 2e) = 0.15 V at pH 4, depending on
260 concentrations of reactants and products. The oxidation of HSO3 by HONO/NO2 however, is
261 complex and proceeds slowly via a free radical mechanism through S and N containing
81 83 84 262 intermediates. In the presence of air, O2 could participate in this process (see below).
263 HONO and NO2 , however, could also produce ⋅OH radicals at significant rates via
264 photolysis, even in optically thick hazes. Estimates made by using the TUV Radiation Model
265 show that the photolysis of HONO (from R1), reaction R3,
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266 HONO + hν (λ ≤ 390 nm) = NO + ⋅OH (R3)
267 is a stronger source of OH radicals than the photolysis of O3(g) even at the [HONO] = 0.01[O3]
268 relative abundances measured during HE.28 This is due to the much larger solar irradiance in the
269 λ ≤ 390 nm actinic range compared with that in the λ ≤ 310 nm range where O3(g) dissociates
1 1 270 into O( D): O3 + hν = O2 + O( D). Our estimates furthermore suggest that OH radical
271 production rates from HONO during hazy and clear days could comparable, because faster
272 production and slower HONO photolysis under haze conditions are compensated by slower
273 production and faster HONO photolysis in clear days. (Table. S1, S.3). We therefore suggest that
274 ⋅OH production from HONO photolysis during HE could play a significant role in sulfate
275 formation.85
86 276 However, since pKa(HONO) ∼ 3, some N(III) will be also present as NO2 (aq) in the pH =
277 3 5 range. The photolysis of NO2 (aq) also produces ⋅OH, but at much lower rates that HONO(g),
278 both because of the integrated molar absorptivity of NO2 ∼ 15 times smaller than that of HONO,
279 and the quantum yield of ⋅OH production: φ(NO2 (aq)→ ⋅OH) ∼ 0.04 is 25 times smaller than
87 280 φ(HONO(g)→ ⋅OH) = 1.0, due to solvent cage effects. Since the reaction of HSO3 with ⋅OH
281 from the photolysis of HONO/NO2 , and presumably as well as the thermal (dark) reaction
282 between NO2 + HSO3 , both proceed via free radicals, O2 is expected to participate in these
283 processes and lead to chain oxidation mechanisms.
284 The preceding considerations leads us to suggest that NO2(g) oxidizes S(IV) indirectly via
285 the free radical mechanism shown below, rather than directly via R0.
+ 286 2 NO2 + H2O = H + NO3 + HONO (i)
287 HONO + hν = NO + ⋅OH (ii)
288 ⋅OH + HSO3 + O2 = HSO4 + HOO⋅ (iii)
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289 ⋅OH + RH + O2 = ROx + HOO⋅ (iv)
290 HOO⋅ + NO = NO2 + ⋅OH (v)
+ 291 NO2 + HSO3 + O2 + H2O = NO3 + HSO4 + H + ⋅OH (R4)
292 Steps (iii) and (iv) as written are not elementary reactions but may proceed via R OO⋅ and H
293 OO⋅ intermediates. The overall stoichiometry of R4 indicates that NO2 and HSO3 produce
294 equimolecular amounts of NO3 and HSO4 , plus an ⋅OH radical that can oxidize HSO3 as well as
295 other species, RH. If ⋅OH reacts with organics RH instead of HSO3 , the HOO⋅ produced in (iv)
296 will regenerate ⋅OH react via (v) as long as there is sufficient NO remaining. Therefore, although
297 NO2 may not be the direct oxidizer of SO2 during HE, the oxidative capacity of the atmosphere
298 will still be determined by initial NO2 concentrations. As noted above, a related thermal chain
299 oxidation mechanism may be initiated by (NO2 + HSO3 ).
300
301 302 303 Figure 6. NO2 reactions and their impact on S(IV) oxidation during haze aerosol events. 304 305 4. 3 Atmospheric Implications
306 This work focused on the particular heterogeneous chemistry that takes place during the
307 severe wintertime haze events (HE) observed in major Chinese cities. We show that the rapid
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308 formation of sulfate aerosol during HE cannot be due to the direct reaction of HSO3 with NO2.
309 Instead, S(IV) could be oxidized indirectly by NO2 via the HONO/NO2 produced in its fast
+ 310 hydrolytic disproportionation: 2 NO2(g) + H2O(l) = H + NO3 (aq) + HONO (R1), a process that
311 is catalyzed by anions at air aqueous aerosol interfaces. The proposed mechanism naturally
312 accounts for the formation of NO3 in the aerosol phase, and the significant concentrations of
313 HONO in the gas phase reported by all field studies. It is expected to be favored at the high
314 relative humidity and high S/V aerosol densities prevalent in HE. We show that the photolysis of
315 HONO and NO2 can be a significant source of OH radicals even under hazy conditions, and
82 316 point out that the thermal reaction between NO2 and HSO3 also proceeds via free radicals,
317 both of which can initiate oxidative chains in the presence of O2. Present findings suggest that
318 future field campaigns should focus on OH radical measurements during winter HE, an issue that
319 has not been properly addressed in the literature. The main insight is that the relevant parameters
320 for a heterogeneous process such as R1 are the surface density S/V of aerosol hazes, which is
321 related to particle size distributions and the fluidity of the interfacial layers, rather than the
322 acidity of aerosol particles. The apparent dependence of sulfate formation rates on ammonia
323 neutralization of aerosol particles may be the result of cross correlations between aerosol acidity
324 and the actual causal parameters. Whereas it remains true that the oxidation of SO2 during HE is
325 driven by primary NO2 emissions, our work, by clarifying the actual mechanism by which this
326 process is initiated, could guide future research efforts and help optimize future air pollution
327 control strategies.
328 Summing up: the direct reaction of NO2(g) with HSO3 on the surface of aqueous aerosols is
329 insignificant in the pH 3 to 6 range. On the surface of aqueous electrolyte solutions, such as
330 those of HE aerosols, NO2(g) is mainly converted to NO3 plus HONO/NO2 via hydrolytic
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331 disproportionation. The implication is that the oxidation of HSO3 during HE coul be mostly due
332 to thermal (dark) and/or photochemical reactions initiated by HONO/NO2 . The acidity/basicity
333 of HE aerosols could be a relevant parameter to S(IV) oxidation rates only if such processes were
334 to depend on aerosol pH.
335 Supporting Information: Additional Figures S1 to S4. Explanatory note S0. TUV calculations
336 S1. Estimated rates of HONO and OH production: S2 and Table S1.
337 Acknowledgments: We acknowledge funding support from National Science Foundation
338 (Grants AC 1238977 & AGS 1744353).
339 * Corresponding author: [email protected]
340
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Aqueous S(IV)
Nebulizer Nebulizer gas N2 mass analyzer
NO2 (g) ------Spraying Chamber
Teflon Tubing Teflon MFC NO2(g) /N2(g)
Figure 1. Schematic diagram of the experimental setup. MFC is the mass flow controller.
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